In a point-to-multipoint communication system such as a PON (Passive Optical Network) system conventionally employed, an OLT (Optical Line Terminal) periodically updates an amount of bandwidth allocation which is information on a transmission grant that is to be given to an ONUs (Optical Network Unit). For efficient use of a bandwidth, the OLT determines an amount of bandwidth allocation according to the traffic condition of the ONUs. When transmitting variable-length data such as Ethernet frames (registered trademark) in the PON system, for example, the length of a grant designated by the OLT and a delimitation of data transmitted from the ONUs may not be at the same point. This generates a time period not used for uplink data transmission, resulting in a loss of a bandwidth. In this regard, in the conventional PON system, the ONU sends a report message to notify a queue length corresponding to the amount of data accumulated in an uplink buffer according to a delimitation in variable-length data. Then, the OLT allocates a grant corresponding to the queue length. In this manner, the OLT can fully use the allocated grant for data transmission, so that the OLT achieves bandwidth allocation without loss (see Patent Literature 1 and Non-Patent Literature 1 cited below).
Patent Literature 1: Japanese Patent Application Laid-open No. H11-146000
Non-Patent Literature 1: “Dynamic Bandwidth Allocation Algorithm for GE-PON” by YOSHIHARA et al., technical report NS2002-17 of the Institute of Electronics, Information and Communication Engineers